In general, metals are easily machined but do not retain their machined form at high temperatures. Ceramics retain their shape at extremely high temperatures, but are brittle and very difficult to machine into a desired shape. Materials scientists have directed a great deal of effort into finding compositions that are easily machined into a desired shape and are stable at extremely high temperatures.
Jeitschko and Nowotny reported the preparation of Ti.sub.3 SiC.sub.2 in 1967. See Montash fur Chem. 98:329-337 (1967). According to their brief description, powdered titanium hydride, silicon and graphite were combined in a self-contained graphite capsule at 2000.degree. C. for about 20 minutes, and then the product was quickly cooled down to 1200.degree. C. The product was extracted to obtain a sample of Ti.sub.3 SiC.sub.2, which was characterized in terms of its crystal structure. The structure was found to be hexagonal and comprised planar Si-layers linked together by TiC octahedra. This characterization allows the theoretical density to be known, which is 4.31 g/cm.sup.3.
A chemical vapor deposition (CVD) approach to Ti.sub.3 SiC.sub.2 was reported by Nickl et al. in 1972, and by Goto et al. in 1987. See J. Less-Common Metals 26:335 (1972) and Mat. Res. Bull. 22:1195-1201 (1987). Each group of researchers used SiCl.sub.4, TiCl.sub.4, CCl.sub.4 and H.sub.2 as source gases. Goto et al. worked at a deposition temperature of 1573-1873.degree. K with the total gas pressure in the CVD furnace fixed at 40 kPa. A monolithic plate of 40 mm by 12 mm by 0.4 mm was obtained with a deposition rate of 200 micrometers/hour.
Okano et al. reported using a mixture of titanium, titanium carbide and silicon powders to form Ti.sub.3 SiC.sub.2. See Proceedings of the 3rd IUMRS International Conference on Advanced Materials, Tokyo, Japan 1993, as published in Advanced Materials '93, I/A: Ceramics, Powders, Corrosion and Advanced Processing, edited by N. Mizutani et al., Trans. Mat. Res. Soc. Jpn., Vol. 14A (Elsevier 1994). The mixture of Okano et al. was compacted uniaxially and then calcined at 1300-1600.degree. C. under a vacuum of 10.sup.-1 Pa for 1 hour. The so-formed Ti.sub.3 SiC.sub.2 was pulverized, pre-formed in a graphite mold and then hot-pressed under a pressure of 45 MPa at 1400.degree. C. for 30 min. This process resulted in vaporization, and consequent loss of silicon. Although the authors attempted to adjust the initial silicon loading to anticipate silicon vaporization, they were not able to prepare pure Ti.sub.3 SiC.sub.2 phase.
In two papers, Pampuch et al. reported forming Ti.sub.3 SiC.sub.2 by igniting a mixture of certain starting materials. See J. European Ceramic Soc. 5:283-287 (1989) and J. Materials Synthesis and Processing 1(2):93-100 (1993). Thus a stoichiometric mixture of titanium, silicon and carbon black was either cold-pressed and the resulting pellet placed in a graphite crucible and heated extremely rapidly from 800.degree. C. to 1020-1080.degree. C., or was placed as a loose powder in a graphite-lined boat and the boat contacted with a heating coil at 1830.degree. C. Under either set of conditions the mixture ignited, causing a very rapid increase in temperature with concomitant formation of Ti.sub.3 SiC.sub.2. The product as formed by either method was porous and contained titanium carbide (10-20%).
Pampuch et al. (1993) also reported that the products from their ignition processes can be ground to a powder, and the powder either pressureless sintered (cold-pressed at 200 MPa followed by sintering at 1450-1700.degree. C. for 1 hour in an argon atmosphere) or hot-pressed (heated to 1250-1500.degree. C. for 15 minutes at a pressure of 25 MPa). These pressure treatments provided Ti.sub.3 SiC.sub.2 containing materials which were 95% of the theoretical density of Ti.sub.3 SiC.sub.2.
After having made many unsuccessful attempts to prepare Ti.sub.3 SiC.sub.2 in a single step, Racault et al. reported a multi-step synthetic procedure which provides Ti.sub.3 SiC with less than 5 atomic % TiC. See J. Materials Science 29:3384-15 3392 (1994). The first step places titanium, silicon and graphite powders in an evacuated tube for 10 hrs. at 1100.degree. C. Thereafter, the product is treated with aqueous hydrogen fluoride to remove TiSi.sub.2 and leave a material consisting of 85% Ti.sub.3 SiC.sub.2 and 15% TiC. The third step is a controlled oxidation at 450.degree. C. in air for 10 hrs, which converts TiC to TiO.sub.2. The fourth and final step dissolves the TiO.sub.2 with a mixture of ammonium sulfate and sulfuric acid at about 100.degree. C.
Arunajatesan and Carim have reported the preparation of Ti.sub.3 SiC.sub.2 from a mixture of titanium, silicon and carbon powders. See J. Am. Ceram. Soc. 78(3):667-672 (1995). The powder mixture was compacted to pellets, optionally arc-melted in an argon atmosphere, and then heated in evacuated quartz tubes at either 900.degree. C. for 24 hr (no Ti.sub.3 SiC.sub.2 formed), 1400.degree. C. for 5 hr (Ti.sub.3 SiC.sub.2 with other phases formed but quartz tubes exploded) or 1200.degree. C. for 100 hr (Ti.sub.3 SiC.sub.2 formed in addition to other phases). The arc melting process caused some loss of silicon and carbon, but yielded samples having superior homogeneity. Treatment of the product with hydrofluoric acid, to leach out titanium suicides, was necessary to prepare final powders with over 99% phase-pure Ti.sub.3 SiC.sub.2.
Arunajatesan and Carim also reported that heating a non-compacted mixture of titanium, silicon and carbon powders in an alumina boat under an argon atmosphere for up to 24 hours at 1270.degree. C. to 1375.degree. C. did not lead to any Ti.sub.3 SiC.sub.2. Furthermore, silicon carbide and titanium carbide were examined as starting materials but did not lead to any Ti.sub.3 SiC.sub.2.
The limited investigation that has been conducted on M.sub.3 X.sub.1 Z.sub.2 phases indicates that they may have commercially valuable properties. There is thus a need in the art for a simple, one-step synthesis of M.sub.3 X.sub.1 Z.sub.2 phases and composites thereof.